With recent development of the color image processing technology, a device for outputting a color image is utilized. Specifically, an image forming device such as a printer for forming a color image on a sheet by using an electrophotography process is utilized. For this color image forming device, there are methods for forming toner images of respective colors directly on a sheet, and for forming toner images of respective colors on an intermediate transfer body and then, transferring the toner images on the intermediate transfer body onto a sheet. The latter is suitable for high speed printing because sheets can be easily fed.
Color image forming devices using such an intermediate transfer body are divided roughly into two types of four-pass type and single-pass type (tandem type). These color image forming devices are disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 9-34269, 10-228188, 2000-147920, 2000-187403, and the like.
By referring to FIGS. 21 and 22, a conventional intermediate transfer body type color image forming method will be described with an example of a single-pass type device. As shown in FIG. 21, image forming units 112-1 to 112-3 are provided for respective colors of yellow (Y), magenta (M), and cyan (C). Note that a black (K) image forming unit is also provided, but omitted for simplicity of description. These image forming units 112-1 to 112-3 have photosensitive drums, and are constituted by disposing cleaning blades, charging units, LED exposure units, and developing units that surround the drums.
In the image forming units 112-1 to 112-3, toner images of the respective colors are formed on the photosensitive drums by a known electrophotography process. The toner images of the respective colors on the photosensitive drums are electrostatically transferred onto a moving intermediate transfer belt 116 in a sequentially superposed manner by applying transfer voltages (referred to as “primary transfer”) Next, the toner image on the intermediate transfer belt 116 is transferred onto an output sheet 120 by a secondary transfer unit (referred to as “secondary transfer”). The toner image on the sheet 12 is fixed by a fixing unit and outputted.
That is, at the time of the primary transfer, onto this intermediate transfer belt 116, a yellow (Y) toner image 130 is transferred, then, a magenta (M) toner image 132 is transferred, and finally, a cyan (C) toner image 134 is transferred. In the cases of a primary color, a secondary color, and a tertiary color, a toner image of one of the three colors, toner images of two of the three colors, and toner images of all of the three colors are transferred, respectively.
Then, this primary transferred image on the intermediate transfer body 116 is transferred onto the medium 120 at one time. The transfer efficiency at this secondary transfer part is, in the case of a primary color, hardly problematic regardless of the charge amount of toner because the deposit amount of the toner is small.
However, in the case of a secondary color, since toner whose amount is twice that of the primary color on the intermediate transfer body, the deposit amount on the intermediate transfer body increases and the secondary transfer efficiency becomes lower. For example, if the deposit amount of the toner becomes doubled with the charge amount thereof kept constant, the toner layer potential becomes quadrupled because the toner layer potential is proportional to the square of the thickness of the toner layer. Basically, in the transfer operation, if a potential having reverse polarity to the potential Vt of the toner layer is applied, the theoretical transfer efficiency becomes 100%. For this purpose, the transfer voltage may be increased, however, since the influence of discharge is exerted, the upper limit will be restricted.
Therefore, when the transfer voltage equal to or less than the upper limit is used, the toner 130 which directly contacts with the belt 116 of the toner images of the secondary color on the belt 116 becomes hard to be transferred. That is, the toner 130 which directly contacts with the belt 116 is strongly adhered to the belt 116, and the toner 132 thereon is weakly adhered to the belt 116.
As shown in FIG. 22, conventionally, since the charge amounts of the toner 130, 132, and 134 of the respective colors are set equal, when the two colors are superposed, the superposition is performed with the toner layer potential and the deposit amount at the same rate. On this account, for example, applying the transfer field with secondary transfer efficiency of 75% is applied, 75% of the toner in the upper toner layer of two colors is secondary transferred.
Thus, the secondary transfer efficiency is difficult to be improved, and a problem arises that the ratio of toner of two colors is varied. Various proposals are made for uniforming the primary transfer efficiency, which is different from the secondary transfer efficiency, among the respective colors. For example, in Japanese Patent Publication (JP-B) No. 1-32981, a method for increasing the charge amounts of the respective colors from the upstream side of the belt toward the downstream side is proposed, and in Japanese Patent Application Laid-Open (JP-A) No. 7-146597, a method for regulating the surface potential, the charge amount of toner, and the thickness of the toner layer before transfer on the most downstream side is proposed.
However, these are methods for uniforming the primary transfer efficiency and the secondary transfer efficiency is not considered. For example, at the time of the primary transfer, since, by the charge of the toner formed on the upstream side on the intermediate transfer body, the primary transfer field is weaken when the next color is transferred, and the transfer efficiency of the next color becomes lower, the proposal is made to make the charge amount of the toner lower toward the upstream side of the intermediate transfer body.
However, by this method, though the primary transfer efficiency becomes uniform for the respective colors, in the secondary transfer, since the lower a layer is on the intermediate transfer body, the smaller a charge amount of the toner thereof is, a problem arises in that the toner in the lower layer becomes more difficult to be secondary transferred.